1. Field of the Invention
This invention relates to compositions and processes for forming a protective coating on metal, particularly zinc, aluminum and/or zinc and/or aluminum alloy, more particularly aluminum and/or aluminum alloy surfaces. In one embodiment of the invention, the primary coating formed is a metal oxide containing conversion coating that solid and adherent, often, although not always colored, provides good corrosion protection to the metal surface, and functions as an excellent base for painting or similar organic based protective coatings. The coatings formed, when applied to substrates of zinc, aluminum and their alloys that contain at least 45% by weight of zinc and/or aluminum, can impart at least as much corrosion protection as do conventional coatings formed by use of hexavalent chromium containing compositions, but the compositions and processes for forming a primary coating according to this invention cause less potential environmental damage because they do not need hexavalent chromium or other identified major pollutants.
In another embodiment of the invention, the corrosion protective effect of a primary coating is further enhanced by a sealing treatment. The sealing treatment is very effective on primary coatings according the other embodiment of the invention, but is not limited thereto; it may be advantageously applied to a wide variety of primary coatings, including those formed by anodization as well as those formed by chemical conversion. The sealing treatment is especially advantageous for surfaces that are not intended to receive any further protective organic based coating such as paint or the like, but is also useful for substrates that are to be further protected in this way.
2. Discussion of Related Art
The published art believed to be most closely related is that described in WO94/00619, particularly Table II thereof and text related thereto. This reference teaches that high quality coatings can be formed on metal substrates by contacting them with aqueous compositions believed to contain cobalt(III) complex anions; the effective compositions are formed by reaction among cobalt(II) salts, carboxylate ions, and various other substances in the presence of an oxidizing agent. Other closely related published art includes U.S. Pat. No. 3,905,838 of Sep. 16, 1975 to Ito and U.S. Pat. No. 5,298,092 of Mar. 29, 1994 to Schriever.
Many of the compositions taught in this related art, although they have avoided the use of hexavalent chromium and other pollutants of the general environment, nevertheless can have adverse environmental impacts on the immediate working area for the process and any workers in this area. For example, many of the formulations previously used include high concentrations of ammonia, which causes at least a severe odor nuisance and possibly a serious health hazard to workers in the vicinity, unless expensive ventilation equipment is installed in the process area. Furthermore, the concentration of ammonia in aqueous solutions is difficult to maintain constant, as is desirable for achieving the most consistent results from the process, at the high levels previously recommended by some related art. In addition, some of the previously recommended compositions contain both nitrite ions and amines, which are generally believed to be readily capable of reacting to form nitrosamines, many of which are known carcinogens.